JP 2005-348193 A (PTL 1) recites an acoustic apparatus, such as a mobile phone or the like, that transmits air-conducted sound and bone-conducted sound to a user. As the air-conducted sound, PTL 1 discloses a sound that is transmitted to the user's auditory nerve by air vibrations, caused by a vibrating object, that are transmitted through the external ear canal to the eardrum and cause the eardrum to vibrate. As the bone-conducted sound, PTL 1 discloses a sound that is transmitted to the user's auditory nerve through a portion of the user's body (such as the cartilage of the outer ear) that is contacting a vibrating object.
In the telephone disclosed in PTL 1, a rectangular plate-shaped vibrating body, formed from a piezoelectric bimorph and a flexible substance, is attached to an outer surface of a housing via an elastic member. PTL 1 discloses that when voltage is applied to the piezoelectric bimorph in the vibrating body, the piezoelectric material expands and contracts in the longitudinal direction, causing the vibrating body to vibrate. Air-conducted sound and bone-conducted sound are transmitted to the user when the user contacts the vibrating body to the auricle.
Apart from a telephone or the like that transmits sound by being held in the hand and pressed against the ear, other devices that could transmit sound based on such a transmission principle include earphones and headphones that are used by being hooked and held somewhere on the human head.